1. Field of the Invention
The present invention relates generally to leak detection and pertains particularly to an improved method and apparatus for detecting very low leak rates.
2. Discussion of the Related Art
Concerns for protecting the environment from pollution has led to the development of leak detection systems particularly for underground storage tanks and pipelines. These conventional leak detection methods and systems employ what is known as the pressure decay method. These systems generally measure the time interval for the pressure in the pipeline to drop from an initial pressure, such as about 10 psi, to a final pressure of about 5 psi.
A typical example of the prior art is illustrated in FIG. 1. Typical fuel storage tank 10 is provided with pump 12 which is typically a submersible pump which supplies pressurized fuel or liquid from the storage tank via pipeline 14 to shut-off valve 16 and fuel nozzle 18 to fuel automobiles and the like. The typical system includes an internal poppet type check valve 22 at the pump which also includes a relief valve. The relief valve acts after the shutoff valve has been closed to relieve the normal pump pressure down to a line pressure of about 15 psi to maintain liquid in the pipeline. When the pump is turned off, the pressure in the pipeline is relieved to 13-15 psi by bleeding excess pressurized fluid back into the underground storage tank. This relief pressure is the starting pressure for the pressure decay systems.
The pressure decay system utilizes a pressure transducer 24 in the line between the pump and the cut-off valve to measure rate of drop in pressure from the 10 psi to 5 psi. A timer is initiated at 10 psi and the time is measured to when the final pressure has dropped to 5 psi, where the timer then stops. The time required for this pressure change is the basis for determining the leak rate. This system has a number of problems.
Among the problems of these prior art devices is that fuel in the underground storage tanks normally contains many small contaminants such as rust and particles of sand. These particles flow through the check valve during fuel dispensing by the station customers and frequently become trapped in the check valve sealing surface causing the check valve to leak. These contaminants in the fuel frequently cause the fluid to leak from the pipeline back into the underground storage tank resulting in a measured pressure drop in the system which will generate a false leak signal.
Another difficulty with the prior art systems is that they also use a relatively large pressure drop as the test interval. The large pressure change required can lead to difficulties in measuring the differences between an actual leak in the pipeline versus an apparent leak. An apparent leak can come from a source such as expansion or contraction in the pipeline due to compressibilities and the significantly larger thermal coefficient of trapped vapor as compared to the fluid. Since the test is also generally run below 13 psi, the effects of compressibility are greatly enhanced and the pressure versus volume relationship is not linear. This makes it very difficult to relate a pressure change to an actual fluid and volume change. This is due to the very large difference in compressibility of trapped vapor and/or flexible piping as compared to the fluid contained within the piping.
The pressure changes due to compressibility and thermal effects are an even greater problem with larger pipelines. A large pipeline is defined in the industry as a three (3) inch line of about 800 ft. in length. The prior art systems are generally ineffective in detecting leaks in these large systems.
One recently developed improved system is disclosed in prior U.S. Pat. No. 5,072,621 entitled PIPELINE LEAK DETECTOR APPARATUS AND METHOD. The system disclosed in that patent measures the flow rate around an in line shut off valve. However, that system does have some limitations that are obviated by the present system.
It is therefore desirable that a system and method for detecting leaks in pipeline be available which overcome the problems of the prior art.